Massive stellar cannibals: How stellar mergers drive mass-loss in extremely massive stars

TL;DR

This work develops a 1D hydrodynamic framework, implemented by coupling a StellarInspiral1D module to MESA, to model circular inspirals of low-mass companions into extremely massive stars ($M_{EMS}>10^3\,M_\odot$). It finds that orbital energy deposited as heat in the EMS envelope excites pulsations that eject about $10$–$30\%$ of the system mass, indicating merger-induced mass loss is non-negligible for EMS formation and evolution. The authors analyze two case studies (EMS of $1000\,M_\odot$ with $10$ and $70\,M_\odot$ companions) to quantify unbound and escaping masses, introduce quantitative mass-loss diagnostics ($M_{unb,max}$, $M_{esc,puls}$, $M_{pul,tot}$), and compare results with analytical estimates and other hydrodynamic simulations. They further provide semi-analytical prescriptions to estimate mass loss across a broader range of eccentricities, highlighting implications for the growth of EMSs and their possible roles as IMBH progenitors and chemical polluters across cosmic history.

Abstract

It has been theorized that the formation of extremely massive and supermassive stars ($>10^3\ {\rm M}_\odot$) could plausibly be the outcome of stellar mergers in low metallicity ($Z<10^{-1}$~Z$_\odot$) and dense ($\gtrsim10^3\ {\rm M}_\odot\ {\rm pc}^{-3}$) stellar environments. These objects remain relevant as they can serve as the progenitors of intermediate-mass black holes and they are also formidable chemical polluter candidates, as evidenced by the peculiar abundances seen across cosmic history. This work investigates merger-induced mass loss in extremely massive stars within a hydrodynamic framework and provides a prescription derived from the simulations to estimate both the mass loss and the outcome of the interaction. We adapted the 1D hydrodynamic, stellar structure, and evolution code MESA to simulate stellar inspirals. In our simulations, we considered stars of $>1000\,\rm M_{\odot}$ with inspiraling companions of $<100$ M$_\odot$; hence, with mass ratios of $<0.1$. As the inspiral progresses, the orbital energy of the system is lost through the hydrodynamic and gravitational drag forces. This energy gets deposited as thermal energy in the extremely massive star's envelope. We find that the total ejected mass is $\sim$10-30$\%$ of the system's mass. Our results point out that most of the energy deposited by the inspiral is used to eject mass. These findings demonstrate that merger-induced mass loss is non-negligible for the considered configurations. Thus, it is an important process to account for when investigating the formation of extremely massive stars and predicting their possible role throughout cosmic history.

Figures (11)

• Figure 1: Inspiral evolution of a 1000 M$_\odot\ $ EMS with a 10 M$_\odot\ $ companion. Left panel (Fig. \ref{['fig:insp_1e3_10a']}): structural and dynamical evolution of the 1000 M$_\odot\ $ EMS during inspiral with a 10 M$_\odot\ $ companion. Top: EMS Kippenhahn diagram, in terms of the radius coordinate with respect to time, with the signed logarithm of the binding energy encoded by the background color; Bottom: same but for $\log(v/v_{\rm esc})$. In both panels cyan dotted regions mark convection zones. The black solid line represents the position of the companion, the black dot marks the merger event. Right panel (Fig. \ref{['fig:insp_1e3_10b']}): Zoom-in on the merger event. Top: EMS radius with $v/v_{\rm esc}$ color-map, the value of the radius at $t=0$ is shown by the red dashed line; Middle: unbound ($M_{\rm unb}$) and escaping ($M_{\rm esc}$) masses. Bottom: surface pressure, its initial value is shown by the red dashed line. In all three plots the time when the merger event is met is marked by the black vertical line.
• Figure 2: Inspiral evolution of a 1000 M$_\odot\ $ EMS with a 70 M$_\odot\ $ companion until the simulation crashes. Left panel (Fig. \ref{['fig:insp_1e3_70a']}): structural and dynamical evolution of the 1000 M$_\odot\ $ EMS during inspiral with a 70 M$_\odot\ $ companion. Top:EMS Kippenhahn diagram, in terms of the radius coordinate with respect to time, with the signed logarithm of the binding energy encoded by the background color; Bottom: same but for $\log(v/v_{\rm esc})$. In both panels cyan dotted regions marks convection zones. The black solid line represents the position of the companion, the black dot marks the merger event. Right panel (Fig. \ref{['fig:insp_1e3_70b']}): zoom-in on the merger event. Top: EMS radius with $v/v_{\rm esc}$ color-map, the initial radius value is shown by the red dashed line; Middle: unbound ($M_{\rm unb}$) and escaping ($M_{\rm esc}$) masses. Bottom: surface pressure, its initial value is shown by the red dashed line. In all three plots the time when the merger event is met is marked by the black vertical line.
• Figure 3: Maximum unbound mass ($M_{\rm unb,max}$, purple triangles pointing down), maximum escaping mass ($M_{\rm esc,max}$, mint triangles pointing up) and predicted total pulsation mass-loss ($M_{\rm pul,tot}$, turquoise diamonds) as a function of companion mass for the three EMS models considered. The gray dashed line indicates the one-to-one relation, a predicted mass-loss below such line means that the EMS gains more mass by the stellar merger than the one it looses through it, and vice versa. The blue dotted line shows the maximum unbound mass estimation if there is no loss of injected energy during the inspiral, i.e. $\alpha=1$ at all times. The black dashed line shows the extrapolated mass-loss prescription from glebbeek2013structure. Analytical predictions from ramirezga2025 are shown with the cherry-hue solid lines.
• Figure 4: Collision outcomes and deposited orbital energy for the interaction between a $1000$ M$_\odot\ $ EMS and a $70$ M$_\odot\ $ companion across initial orbital configurations. The EMS structural profile after the relaxation steps is used for trajectory integration. The initial semi latus rectum ranges from $10$ to $10^5$ R$_\odot$ and the eccentricity from $0$ to $2$. The EMS radius is indicated by the gray dashed line. Left plot: classification of collision outcomes (A to D, see Sec. \ref{['sec:across_ecc']}) as a function of the initial semi latus rectum and eccentricity. Right plot: deposited orbital energy for mergers and scatterings in the absence of external interactions.
• Figure 5: Same as Fig. \ref{['fig:col_class']}, but using the EMS profile at the end of the hydrodynamic collision simulation for the interaction between a $1000$ M$_\odot\ $ EMS and a $70$ M$_\odot\ $ to determine the possible outcomes of a second collision. The EMS has expanded to a radius of $\sim 5\times 10^4$ R$_\odot$, shown by the gray dashed line in both panels. Left plot: collision outcome classification, right plot: deposited orbital energy for the same configurations.